We describe the organization of the paper.
The second section contains the definition and basic properties.
In the third section discusses some small algebras and exceptions.
The next section deals with consistency, bases and dimensions in some detail.
In Section 5 we prove Theorem REF . The final section
studies Auslander-Reiten components and contains the proof of Theorem
REF .
For further background and motivation, we refer to the introductions of [1]}, [2]} or [3]}.
